Converters for establishing a digital representation of an analog signal, i.e. A/D-converters, are well-known in the art. Several types of A/D-converters exist, and among these, the class of time quantization converters or pulse modulation converters, e.g. PWM modulating A/D-converters and delta-sigma A/D-converters has proven very advantageous. This is among other things because up-to-date technology enabling very fast processing facilitates more accurate precision in time than in amplitude.
A problem with time quantization converters is quantization errors introduced by the sampler or quantizer whenever the actual edge of a pulse does not happen to coincide with a clock edge. Improved time quantization A/D-converters therefore comprise a feedback around the sampler or quantizer for suppressing the quantization error. Examples of PWM A/D-converters are described in the applicant's PCT patent applications WO 2005/029707 A1 and WO 2005/029708 A1, hereby incorporated by reference in their entirety.
Also the feedback time quantization A/D-converters suffer, however, from a problem, namely that the D/A-conversion or corresponding action required to feedback the digital output to compare with the analog input introduces time jitter errors in the feedback path. This time jitter may e.g. derive from the clock controlling the feedback path D/A-converter. And because the errors are introduced in the feedback path itself, they are not suppressed by the loop, and directly affect the performance of the A/D-converter.
An object of the present invention is to obtain a time quantizing A/D-converter with feedback, which features attenuation of errors, in particular jitter, introduced in the feedback path.